This invention relates to a method for preparing endblocked methylpolysiloxane fluids having improved thermal stability and to the thermally improved methylpolysiloxane fluids obtained thereby. In one aspect this invention relates to a method for preparing a titanium-, zirconium-, or hafnium-containing methylpolysiloxane thermal-stability additive and to a method for stabilizing a methylpolysiloxane fluid therewith. In another aspect this invention relates to a heat-transfer system comprising an improved heat-transfer fluid.
Herein the general term "endblocked" refers to hydrocarbon endblocking groups such as methyl endblocks such as are present in trimethylsiloxane endblocks and does not include hydrolyzable endblocking groups such as hydroxy endblocks and alkoxy endblocks.
Although organopolysiloxanes, especially methylpolysiloxanes are well known for their thermal stability, considerable effort has been devoted by the prior art to obtaining improvements therein. Instability of organopolysiloxane at high temperature is related to reactions of the silicon-bonded organic radicals, such as cleavage and oxidation, which often lead to crosslinking and gelation of the organopolysiloxane; and to rearrangement reactions of siloxane linkages, which often leads to depolymerization of the siloxane and to the formation of lower molecular weight siloxanes. A large part of the prior art relates to stabilizing organopolysiloxanes to radical cleavage and oxidation at high temperature. This invention is related to a method of improving the stability of an endblocked methylpolysiloxane toward siloxane rearrangement at high temperature in the absence of more than trace amounts of moisture and to the compositions obtained therefrom.
Organopolysiloxanes are known to readily undergo siloxane rearrangement in the presence of acidic and alkaline catalysts to produce a different arrangement of siloxane linkages. For example, a trimethylsiloxane-endblocked polydimethylsiloxane will give rise to cyclopolydimethylsiloxanes such as the corresponding cyclotrisiloxane and cyclotetrasiloxane and to shorter chain trimethylsiloxane-endblocked polydimethylsiloxanes, including the shortest species, i.e. hexamethyldisiloxane, when heated in the presence of a rearranging catalyst such as sulfuric acid or sodium hydroxide.
Many organopolysiloxanes in which no catalyst has been intentionally added also undergo siloxane rearrangement to some extent at high temperature. This rearrangement is thought to be due to the presence of silanol in the siloxane. For example, Rode, et al., Vysokomol. soyed. All: No. 7, 1529-1538, 1969 have studied the thermal degradation and stabilization of polydimethylsiloxane in the absence of oxygen and have found that the rate of thermal degradation of a hydroxy-endblocked polydimethylsiloxane toward cyclotrisiloxane formation could be decreased by reacting the hydroxyl endgroups with acetylacetonates of copper, iron or zirconium in m-cresol. They have also found that the thermal stability of the hydroxy-endblocked polydimethylsiloxane fluids could be improved merely by adding thereto certain additives such as acetylacetonates of aluminum, zinc, cobalt, copper, iron, and zirconium and titanium-tetrabutoxylate; however, intensive crosslinking and gelation of the treated siloxane also occurred at low temperatures. Rode, et al. also noted that a trimethylsiloxane-endblocked polydimethylsiloxane also undergoes siloxane rearrangement to form the cyclotrisiloxane but they proposed no solution for this problem.
Britt, U.S. Pat. No. 3,759,970 stabilizes polysiloxane fluids by replacing impurity groups, such as SiCl, SiH, and SiOH with a fluoride group. Britt speaks to the problem of oxidative instability as evidenced by gelation of the polysiloxane fluid in the air at high temperature but states nothing about inhibiting siloxane rearrangement in the absence of air.
Organic titanium and zirconium compounds have found use in organopolysiloxane compositions. For example, Ceyzeriat, et al., U.S. Pat. No. 3,151,099 employ large amounts of titanium alkoxides or zirconium alkoxides as a component in a moisture-curing composition. Brown, et al., U.S. Pat. No. 3,745,129 use numerous siloxane organometallocene compounds to protect polydiorganosiloxane fluids against oxidation. Hunter, et al., U.S. Pat. No. 2,728,736 use large amounts of zirconium alkoxides in organopolysiloxane compositions which are suitable for treating leather. McNulty, et al., U.S. Pat. No. 2,687,388 use small amounts of a zirconium salt of an organic acid which is soluble in polyorganosiloxanes to function as a hardening agent for the polyorganosiloxane. Swiss, U.S. Pat. No. 2,465,296 uses minor amounts of certain metal chelates to stabilize organosilicon oxide polymers against oxidation at high temperatures. Swiss further teaches that the solution of metal chelate and organosilicon oxide polymer should be heated to 200.degree.-250.degree. C. to impart much better oxidation resistance to the composition. It is also known that certain cerium-containing heat-stability additives for organopolysiloxanes which have improved resistance to precipitation, can be prepared by reacting certain alkali metal siloxanolates with cerium salts and at least one organic carboxylic acid salt or alkoxy derivative of zirconium, titanium, or iron.
Thus, while the above prior art teaches that organic titanium or zirconium compounds may be used in organopolysiloxanes for many reasons, including stabilization of hydroxy-endblocked polydimethylsiloxanes toward siloxane rearrangement, nothing is taught therein regarding the stabilization of hydrocarbon-endblocked methylpolysiloxanes toward thermal siloxane rearrangement in the absence of moisture using only said titanium or zirconium compounds.